In a conventional acoustic device for performing stereophonic sound reproduction, if left and right speakers are disposed without sufficient space therebetween, dimensional sound cannot be perceived. In order to produce dimensional sound, a difference signal (L-R) is extracted from left and right channel sound signals L and R. Then, a signal whose level and phase are controlled is added to the left channel sound signal L, while a signal of opposite phase relative to the signal having the controlled level and phase is added to the right channel sound signal R.
For example, a sound image enhancement circuit 1' has a structure shown in FIG. 23. In this structure, the left channel sound signal L and the right channel sound signal R are input to left and right channel input terminals 2L and 2R, respectively. The left channel sound signal L is sent to an adder 6L, while a signal of opposite phase relative to the left channel sound signal L is output to an adder 3. Similarly, the right channel sound signal R is sent to the adder 3 and an adder 6R.
In the adder 3, after a difference signal (L-R) is generated based on the input left and right channel sound signal L and R, the level of the difference signal (L-R) is attenuated by a predetermined amount by an attenuator 4 with an attenuation coefficient A. Then, a signal [(L-R).multidot.A] is sent to a phase shifter 5.
In the phase shifter 5, the phase of the input signal [(L-R).multidot.A] is shifted by .PHI., and a signal [(L-R).multidot.A].angle..PHI. (where .angle. represents the phase) is sent to the adder 6L. At this time, a signal -[(L-R).multidot.A].angle..PHI. of opposite phase relative to the input signal [(L-R).multidot.A].angle..PHI. is sent to the adder 6R. In the adder 6L, an output of the phase shifter 5 and the left channel sound signal L are added, and a signal [L+((L-R).multidot.A).angle..PHI.] is output as a reproduced sound output from an output terminal 7L. Similarly, in the adder 6R, a signal of opposite phase relative to the output of the phase shifter 5 and the right channel sound signal R are added, and the resulting signal [R-((L-R).multidot.A).angle..PHI.] is output as a reproduced sound output from an output terminal 7R.
In order to simplify the explanation, assume that the right channel sound signal R is zero. Then, a signal [L(1+A.angle..PHI.)] is output as a reproduced sound output from the output terminal 7L, while a signal (-LA.angle..PHI.) is output as a reproduced sound signal from the output terminal 7R. This is explained by a vector diagram shown in FIG. 24. For the sake of convenience, the vectors of the reproduced sound outputs from the output terminals 7L and 7R are indicated as 7L and 7R, respectively, in FIG. 24.
When the vectors 7L and 7R are combined, a virtual speaker 10L' is located on a line connecting speakers 10L and 10R along the direction of the synthetic vector as shown in FIG. 24.
Similarly, with respect to the right channel sound signal, assuming that the left channel sound signal L is zero, when the vectors 7L and 7R are combined, a virtual speaker 10R' is located on a line connecting the speakers 10L and 10R along the direction of the synthetic vector. Such a placement of the virtual speakers 10L' and 10R' is achieved by adjusting the attenuator 4 and the phase shifter 5.
As described above, the sound image enhancement circuit 1' performs analog processing using an analog circuit. However, it is also possible to obtain similar results by performing digital processing using a DSP (Digital Signal Processor).
A virtual sound source is generated on the basis of a transfer function. In this case, the transfer function is given according to the order of an FIR (Finite Impulse Response) filter, processed by the DSP. Referring now to FIG. 25, the following description discusses sound image enhancement on the basis of a transfer function.
How the virtual speaker 10L' is realized with the use of the two speakers 10L and 10R will be explained with reference to FIG. 25. The explanation is made by denoting the sound sources in the L channel and R channel as S.sub.L and S.sub.R, respectively, the transfer function when sounds from the speakers 10L and 10R fall on each ear of a listener as H.sub.AL, H.sub.AR, H.sub.BL and H.sub.BR, and the transfer function when a sound from the virtual speaker 10L' falls on the left ear of the listener as H.sub.R and H.sub.L. In addition, assuming that only the L-channel sound source S.sub.L is present as the sound signal (S.sub.R =0), signals input to the speakers 10L and 10R are L and R, respectively, the level of sound pressure when sounds from the speakers 10L and 10R fall on the left ear is E.sub.L and that the level of sound pressure when the sounds fall on the right ear is E.sub.R, the following equations are established. EQU E.sub.L =L.multidot.H.sub.AL +R.multidot.H.sub.BL ( 1) EQU E.sub.R =L.multidot.H.sub.AR +R.multidot.H.sub.BR ( 2)
Moreover, assuming that the level of sound pressure when a sound from the virtual speaker 10L' falls on the left ear is E.sub.L ' and that the level of sound pressure when the sound falls on the right ear is E.sub.R ', the sound pressure is given: EQU E.sub.L '=S.sub.L .multidot.H.sub.L ( 3) EQU E.sub.R '=S.sub.L .multidot.H.sub.R ( 4)
In this case, in order to achieve a virtual speaker based on the sounds from the speakers 10L and 10R, it is necessary to satisfy the following equations at the positions of the ears of the listener. EQU E.sub.L '=E.sub.L and E.sub.R '=E.sub.R
Next, when the listener is equidistant from the speakers 10L and 10R, the transfer functions from the speakers 10L and 10R become symmetrical between left and right with respect to the position of the listener. Since the equations H.sub.AL =H.sub.BR and H.sub.AR =H.sub.BL are established, the signals L and R input to the speakers 10L and 10R are given: EQU R=S.sub.L .multidot.(H.sub.L .multidot.H.sub.AR -H.sub.R .multidot.H.sub.AL)/(H.sub.AR .multidot.H.sub.AR -H.sub.AL .multidot.H.sub.AL) (5) EQU L=S.sub.L .multidot.(H.sub.L .multidot.H.sub.AL -H.sub.R .multidot.H.sub.AR)/(H.sub.AR .multidot.H.sub.AR -H.sub.AL .multidot.H.sub.AL) (6)
Suppose that EQU H0=(H.sub.L .multidot.H.sub.AR -H.sub.R .multidot.H.sub.AL)/(H.sub.AR .multidot.H.sub.AR -H.sub.AL .multidot.H.sub.AL) EQU H1=(H.sub.L .multidot.H.sub.AL -H.sub.R .multidot.H.sub.AR)/(H.sub.AR .multidot.H.sub.AR -H.sub.AL .multidot.H.sub.AL),
equations (5) and (6) above are written: EQU R=S.sub.L .multidot.H0 (7) EQU L=S.sub.L .multidot.H1 (8)
By outputting the signals L and R represented by the above-mentioned transfer functions from the speakers 10L and 10R, the virtual speaker 10L' is realized.
The transfer functions are actually given by obtaining the order of (the number of steps in) the FIR filter using, for example, a window function with respect to the results of measurement at the positions of the speakers 10L and 10R and the position of the virtual speaker 10L'. The order of the FIR filer is usually obtained as follows. Suppose that the order is N, the sampling frequency is f.sub.s, an attenuation band is .DELTA.f, and the coefficient is D (where D is between 0.9 and 1.3), EQU N=[[(f.sub.s /.DELTA.f).multidot.D+1]]
where [[x]] is a minimum odd integer larger than x.
For example, if f.sub.s =48 kHz, .DELTA.f=200 Hz, and D=1, the order N becomes 243. However, in general, since the window function is used, the order is decreased and the order of the FIR filter is sufficiently utilized with 128 steps. For the convolutional operation of the FIR filter, since the operation is carried out twice for each channel, an operation including more than 128.times.2=256 steps in total is required. By changing the coefficient of the convolutional operation of the FIR filter, the virtual speaker is placed in a desired position. The structure according to the above explanation is shown in FIG. 26. An FIR filter 35L corresponds to equation (7), and an FIR filter 36L corresponds to equation (8). FIR filters 35R and 36R correspond to the case where only the R-channel sound signal R is present as a sound signal (S.sub.L =0), and a detailed explanation thereof will be omitted here.
In a conventional art, in order to simulating the perception of a sound field at a live performance (in order to obtain a sound field simulation of Concert Hall, Nightclub, or Stadium), reverberation signals are generated based on input sound signals using a delay circuit, added to the input sound signals, and then reproduced by two front speakers. In order to more faithfully simulate the perception of the live performance, two rear speakers may be provided at the back in addition to the two front speakers so that the reverberation signals are reproduced by the rear speakers.
However, with this conventional art using a phase shifter, the sound sources only spread on a line connecting the left and right speakers. Since a sound image can not spread to the back of the listener, the conventional art fails to simulate the perception of a live performance.
Moreover, high frequency sounds do not spread, and thus the resulting sounds have a rather monaural sound quality. Therefore, with the conventional art, it is necessary to provide additional speakers at the back of the listener in order to more faithfully simulate the perception of a live performance.
Furthermore, when performing digital processing using a DSP, virtual speakers are located in desired positions by reproducing the resulting outputs of the FIR filter. Namely, it is possible to provide the virtual speakers at the back of the listener and to satisfactorily simulate the perception of a live performance. However, as described above, in order to perform the operation of 256 steps for each channel by the DSP, it is necessary to use a plurality of extremely high-speed DSPs. However, since such an extremely high-speed DSP is fairly expensive, the cost of the apparatus on the whole becomes very expensive.
In addition, with a conventional art related to simulating the perception of a live performance, although the effect of reverberation sounds is produced by providing only two speakers at the front, a satisfactory perception of a live performance can hardly be simulated. If four speakers are installed at the front and back, it is necessary to determine the installation positions of the rear speakers with precision. Besides, since the two rear speakers are additionally provided, the structure of the apparatus becomes complicated. Consequently, such an apparatus has not widespread among the ordinary families.